Distillation has been generally employed for separating methanol from a methanol-containing organic mixture solution. However, a common distillation method has the problem that it requires an enormous amount of thermal energy during vaporizing a solution for separation in a distillation column.
Furthermore, in the case of separating an azeotropic mixture which is inseparable by the common distillation method or a mixture of organic materials having close boiling points, an azeotropic distillation method containing very complicated many steps or extractive distillation method must be employed.
Recently, as a method for separating an organic mixture solution which is different from a conventional distillation separation method, there has been proposed a method wherein an organic mixture solution is vaporized and the vapor is fed to the feeding side of a gas separation membrane while allowing a particular organic component to selectively permeate toward the permeate side of the gas separation membrane.
For separation of a mixed organic vapor using a polyimide asymmetric gas separation membrane, Patent Reference No. 1 has described a method for separating a lower alcohol using a gas separation membrane made of a heat-resistant aromatic polymer. Patent Reference No. 1 has described separation of a mixed vapor containing methanol and methyl t-butyl ether or that containing methanol and toluene using an asymmetric hollow fiber membrane for gas separation, which is produced by heating a polyimide made of biphenyltetracarboxylic acid, diaminodiphenyl ether and di(aminophenoxy)benzene (which is identical to bis(aminophenoxy)benzene) at 160° C.